It is well known that film pictures, such as movies, for example, are signals having a 24 Hz frame frequency, while the NTSC system, which is widely adopted in e.g., Japan and the U.S.A., employs interlaced scanning format signals with a 30 Hz frame frequency.
In general, broadcast stations convert the film pictures from a progressive scanning signal format with a frame frequency of 24 Hz to an interlaced scanning signal format with a frame frequency of the 30 Hz, by performing a 3:2 pull-down conversion process before broadcasting the film pictures.
At the receiving side, the received signals are displayed by processing in similar ways as the conventional NTSC system TV signals or by processing a frame frequency conversion as disclosed in the Japanese Patent Application; Tokkai-Hei 7-95441.
In recent years, large-sized screen projection TV sets using LCD technology have been placed in the market with display units generally employing progressive scanning formats. Further, because of the appearance of large-sized screens, there is a demand for higher quality reproduced images.
For instance, when NTSC TV signals (i.e., interlaced scanning format signals with 30 Hz frame frequency) are inputted, it becomes possible to achieve the higher quality display by generating and displaying the progressive scanning format signals using the progressive scanning conversion method disclosed in the Japanese Patent Application; Tokkai-Hei 4-157886.
On the other hand, in DVD systems, which have recently come into the market, film pictures are recorded on DVD discs at the 24 Hz frame frequency. The recorded signals, however, are reproduced by DVD players as a 3:2 pull-down converted 30 Hz interlaced scanning format signals.
When displaying signals on a display unit using LCD technology, it is possible to obtain high quality reproduced images using the frame frequency conversion technique disclosed in the Japanese Patent Application; Tokkai-Hei 7-95441 as described above.
However, the reproduced images are not always the images of frame signals having a 24 Hz frequency and the signals may be contaminated with NTSC system TV signals. Although image memory is indispensable for the progressive scanning and conversion processes of these signals, the cost of the occupied image memory is high. Therefore, it is desirable to share the memory among a plurality of applications.
In such cases, when the input signal to be stored in memory has been transposed from the frame frequency 24 Hz video signals to the NTSC system TV signals (i.e., interlaced scanning format signal with 30 HZ frame frequency), the reproduced images may be disturbed. Therefore, the output of the memory has been conventionally masked for a prescribed period in order to conceal the disturbances of the reproduced images.